Catalyst and process for preparing isoolefins

ABSTRACT

A catalyst, useful in the preparation of isoolefins and containing 0.1 to 20% by mass of an alkali metal oxide, an alkaline earth metal oxide and mixtures thereof; 0.1 to 99% by mass of aluminum oxide; and 0.1 to 99% by mass of silicon dioxide, is prepared by a) treating an aluminosilicate with an aqueous alkali metal salt solution, an alkaline earth metal salt solution and mixtures thereof, under acidic conditions, to obtain a treated aluminosilicate; and b) calcining the treated aluminosilicate, to obtain the catalyst.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a catalyst for cleaving alkyltert-alkyl ethers or tertiary alcohols to isoolefins and alcohol orwater.

2. Description of the Related Art

Isoolefins, for example isobutene, are important intermediates for thepreparation of a multitude of organic compounds. Isobutene is, forexample, a starting material for the preparation of butyl rubber,polyisobutylene, isobutene oligomers, branched C₅ aldehydes, C₅carboxylic acids, C₅ alcohols and C₅ olefins. It is also used as analkylating agent, especially for the synthesis of tert-butylaromatics,and as an intermediate for obtaining peroxides. In addition, isobutenecan be used as a precursor for methacrylic acid and its esters.

In industrial streams, isoolefins are usually present together withother olefins and saturated hydrocarbons with the same number of carbonatoms. The isoolefins cannot be removed in an economically viable mannerfrom these mixtures with physical separating methods alone.

For example, isobutene is present in typical industrial streams togetherwith saturated and unsaturated C₄ hydrocarbons. Owing to the smallboiling point difference and the small separating factor betweenisobutene and 1-butene, isobutene cannot be removed from these mixturesin an economically viable manner by distillation. Isobutene is thereforefrequently obtained from industrial hydrocarbons by converting isobuteneto a derivative which can be removed easily from the remaininghydrocarbon mixture, and by dissociating the isolated derivative back toisobutene and derivatizing agent.

Typically, isobutene is removed from C₄ cuts, for example the C₄fraction of a steamcracker, as follows. After removal of the majority ofthe polyunsaturated hydrocarbons, mainly the butadiene, byextraction/extractive distillation or selective hydrogenation to givelinear butenes, the remaining mixture (raffinate I or selectivelyhydrogenated crack-C₄) is reacted with alcohol or water. Isobutene formsmethyl tert-butyl ether (MTBE) when methanol is used, ethyl tert-butylether (ETBE) when ethanol is used and tert-butanol (TBA) when water isused. After they have been removed, these derivatives can be cleaved toisobutene in a reversal of their formation.

The cleavage of alkyl tert-butyl ethers (ATBE) to the correspondingisoolefins and alcohols and the cleavage of tertiary alcohols to thecorresponding isoolefins and water can be performed in the presence ofacidic or basic catalysts in the liquid phase or gas/liquid mixed phaseor in the pure gas phase.

The cleavage in the liquid phase or gas/liquid phase has thedisadvantage that the products formed, dissolved in the liquid phase,can enter into side reactions more easily. For example, the isobuteneformed in the cleavage of MTBE can form undesired C₈ and C₁₂ componentsas a result of acid-catalysed dimerization or oligomerization. Theundesired C₈ components are mainly 2,4,4-trimethyl-1-pentene and2,4,4-trimethyl-2-pentene. In addition, some of the methanol formed inthe cleavage is converted to dimethyl ether with elimination of waterparticularly over basic catalysts. When the reaction is not performedunder pressures above the saturation vapour pressure of the reactionmixture, in order to counteract these problems, an additional solvent isnecessary.

In the gas phase, the formation of by-products as a result of reactionof the cleavage products with themselves can be suppressed owing totheir lower concentrations in comparison to the cleavage in the liquidphase. However, other side reactions can occur owing to the relativelyhigh cleavage temperatures. In the gas phase cleavage, catalysts aretherefore required which catalyse the cleavage of tertiary alkyl ethersor tertiary alcohols to isoolefin and alcohol or water with very highselectivity, but do not promote any side reactions, for example C-Ccleavage or dehydrogenation and C-C coupling reactions or etherformation of the alcohols formed. Moreover, these catalysts shouldenable high space-time yields and have a long lifetime. In addition,cleavage of the reactant with maximum selectivity for the isoolefinformed at a pressure of greater than 0.3 MPa is desirable.

The catalysts described in the literature for the gas phase cleavage ofalkyl tert-alkyl ethers (ATAE) and tertiary alcohols to thecorresponding isoolefins and alcohol or water are a multitude ofcatalysts. This is true in particular for catalysts which are utilizedfor the cleavage of methyl tert-butyl ether (MTBE).

The catalysts used are usually metal oxides having an empirical formulaof M_(a)O_(x), mixed metal oxides with empirical formulaeM_(a)M_(b)M_(n)O_(y), especially those which contain M=Si or M=Al, acidson metal oxide supports or metal salts.

U.S. Pat. No. 4,254,290 describes, as cleavage catalysts, for example,SiO₂/Al₂O₃ or WO₃/Al₂O₃. U.S. Pat. No. 4,320,232 and U.S. Pat. No.4,521,638 claim, for the cleavage of tertiary ethers, catalystsconsisting of phosphoric acid on supports. Aluminum oxide on silica gelis utilized as a cleavage catalyst in U.S. Pat. No. 4,398,051. In thetwo U.S. Pat. No. 4,357,147 and U.S. Pat. No. 5,254,785, zeolites areused for the same purpose.

In JP 59010528, the cleavage catalyst used is sulphated titanium dioxideor zirconium dioxide. Ethers are cleaved in U.S. Pat. No. 5,607,992 byusing a zirconium oxide/cerium oxide catalyst, in U.S. Pat. No.6,124,232 by using zirconium oxide/tungsten oxide, in U.S. Pat. No.6,162,757 by using a mixed oxide of zirconium and rare earths.

WO 2005-066101 claims a catalyst with the general empirical formulaX_(m)Y_(n)Z_(p)O_(q) where X is at least one element of the fourth groupof the Periodic Table of the Elements, Y is at least one metal from thethird and/or sixth group and Z is at least one element from the seventh,eighth or eleventh group.

JP 1993-229965 claims a catalyst with the empirical formulaSi_(a)X_(b)Y_(c)Z_(d)O_(e). (Here, Si and O in each case are silicon andoxygen; X is at least one element which is selected from the groupconsisting of titanium and zirconium; Y is an element which is selectedfrom the group consisting of magnesium and calcium; Z is at least oneelement which is selected from the group consisting of sodium,potassium, chlorine and sulphur; a, b, c, d and e indicate the atomicratio of the individual elements. When a=1, b=0.001 to 10, c=0.0001 to5, d=0 to 1; e is the number of oxygen atoms needed to satisfy thevalency of the individual constituents mentioned above.)

U.S. Pat. No. 5,171,920 describes, in Example 4, the preparation of acleavage catalyst which, in a formal sense, contains the componentssilicon dioxide, aluminum oxide and magnesium oxide. The preparation isdone in such a way that silicon dioxide is first saturated/impregnatedwith an aqueous magnesium nitrate solution, and an intermediate dryingis followed by a further saturation/impregnation with an aqueousaluminum nitrate solution. Subsequently, predrying is followed bycalcination.

EP 0 589 557 claims, inter alia, a cleavage catalyst which consists, ina formal sense, of magnesium oxide, aluminum oxide and silicon dioxide.In its preparation, an aluminosilicate is impregnated in a first stepwith an aqueous magnesium salt solution in such a way that, during theimpregnation, the pH of the impregnation solution can be adjusted to apH of 7 to 11 by adding a base. In order to obtain particularly activeand selective catalysts, impregnation times of over 200 h are requiredin some cases.

In the cleavage of alkyl tert-alkyl ethers or tertiary alcohols toisoolefin and alcohol, the known catalysts have one or more of thefollowing disadvantages: low selectivity for the target products, use ofhigh temperatures in the cleavage, in some cases above 500° C., shortlifetimes of the catalysts, and complicated and hence costly preparationof the catalyst.

SUMMARY OF THE PRESENT INVENTION

It is therefore an object of the present invention to provide a cleavagecatalyst which does not have one or more of these disadvantages.

This and other objects have been achieved by the present invention thefirst embodiment of which includes a process for preparing a catalyst,comprising:

-   -   a) treating an aluminosilicate with a member selected from the        group consisting of an aqueous alkali metal salt solution, an        alkaline earth metal salt solution and mixtures thereof, under        acidic conditions, to obtain a treated aluminosilicate; and    -   b) calcining said treated aluminosilicate, to obtain said        catalyst which comprises 0.1 to 20% by mass of a member selected        from the group consisting of an alkali metal oxide, an alkaline        earth metal oxide and mixtures thereof; 0.1 to 99% by mass of        aluminum oxide; and 0.1 to 99% by mass of silicon dioxide.

In another embodiment, the present invention relates to a catalyst,comprising:

(i) 0.5 to 20% by mass of a member selected from the group consisting ofan alkali metal oxide, an alkaline earth metal oxide, and mixturesthereof;

(ii) 4 to 30% by mass of aluminum oxide; and

(iii) 60 to 95% by mass of silicon dioxide.

In yet another embodiment, the present invention relates to a processfor preparing an isoolefin having 4 to 6 carbon atoms of the formula I,

said process comprising:

-   -   catalytic gas phase cleavage of a starting compound of the        formula II at a temperature of 110 to 450° C. and a pressure of        0.1 to 2 MPa

to give a compound of the formula I and a compound of the formula III

R—OH  III

wherein, in the formulae I to III,

R is H or an alkyl radical having 1, 2 or 3 carbon atom(s),

R¹ is H or a methyl or ethyl radical, and

R² and R³ are each, independently methyl or ethyl radicals,

R² and R³ may be the same or different,

wherein a catalyst used in the gas phase cleavage is a catalyst asabove.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

It has now been found that, surprisingly, mixed oxide catalysts whichconsist, in a formal sense, of aluminum oxide, silicon dioxide and atleast one oxide from the group of the alkali metal or alkaline earthmetal oxides, especially magnesium oxide, and which are preferablyprepared by impregnating an aluminosilicate at least once with an acidicaqueous solution which contains at least one alkali metal and/oralkaline earth metal compound, and subsequent calcination, cleave alkyltert-alkyl ethers (ATAE) and tertiary alcohols with very highselectivities to the corresponding isoolefins and have very longlifetimes. The impregnation can be effected, for example, by simplymixing the aluminosilicate with the acidic aqueous impregnation solutionwithout any need for additional pH monitoring and without particularlylong impregnation times being necessary. This finding is surprising,since EP 0 589 557 A2 explicitly points out that, during theimpregnation, the pH in the supernatant solution has to be kept in therange of 7 to 11 by addition of bases, and that long impregnation timesare required in order to obtain catalysts with high activity and lowtendency to form by-products.

The present invention therefore provides a process for preparing acatalyst which, in a formal sense, comprises 0.1 to 20% by mass ofalkali metal and/or alkaline earth metal oxide, 0.1 to 99% by mass ofaluminum oxide and 0.1 to 99% by mass of silicon dioxide, which ischaracterized in that it comprises the steps of treating analuminosilicate with an aqueous alkali metal and/or alkaline earth metalsalt solution under acidic conditions and calcining the aluminosilicateimpregnated with aqueous alkali metal and/or alkaline earth metal saltsolution. The amount of alkali metal and/or alkaline earth metal oxideincludes all values and subvalues therebetween, especially including0.5, 1, 2, 3, 4, 5, 10, 12, 14, 16 and 18% by mass. The amount ofaluminum oxide includes all values and subvalues therebetween,especially including 0.5, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55,60, 65, 70, 75, 80, 85, 90 and 95% by mass. The amount of silicondioxide includes all values and subvalues therebetween, especiallyincluding 0.5, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70,75, 80, 85, 90 and 95% by mass.

The present invention likewise provides a catalyst which, in a formalsense, comprises alkali metal and/or alkaline earth metal oxide,aluminum oxide and silicon dioxide, which is characterized in that thecatalyst, in a formal sense, has a content of alkali metal and/oralkaline earth metal oxides of 0.5 to 20% by mass, a content of aluminumoxide of 4 to 30% by mass and a content of silicon dioxide of 60 to 95%by mass, and also a process for preparing isoolefins having 4 to 6carbon atoms of the formula I

by catalytic gas phase cleavage of a starting compound of the formula II

to give a compound of the formula I and a compound of the formula III

R—OH  III

where, in the formulae I to III, the R radical is H or an alkyl radicalhaving 1 or 2 carbon atom(s), the R¹ radical is H or a methyl or ethylradical, and the R² and R³ radicals are each methyl or ethyl radicals,where the R² and R³ radicals may be the same or different, at atemperature of 110 to 450° C. and a pressure of 0.1 to 2 MPa, which ischaracterized in that the catalyst used in the gas phase cleavage is aninventive catalyst or a catalyst prepared in accordance with the presentinvention. The content of alkali metal and/or alkaline earth metaloxides includes all values and subvalues therebetween, especiallyincluding 1, 2, 4, 6, 8, 10, 12, 14, 16, 18 and 20% by mass. The contentof aluminum oxide includes all values and subvalues therebetween,especially including 5, 6, 8, 10, 12, 14, 15, 16, 18, 20, 22, 24, 25,26, 28 and 30% by mass. The content of silicon dioxide includes allvalues and subvalues therebetween, especially including of 60, 65, 70,75, 80, 85, 90 and 95% by mass. The temperature includes all values andsubvalues therebetween, especially including 150, 200, 250, 300, 350 and400° C. The pressure includes all values and subvalues therebetween,especially including 0.5, 1, 1.5 MPa.

The preparation of isoolefins by gas phase cleavage of ATAE or tertiaryalcohols using the inventive catalyst has several advantages: even inthe case of conversions of the feedstocks of over 70%, the correspondingisoolefins are formed in selectivities of over 99%. In the case ofcleavage of ATBE, the selectivities for the ethers formed from theeliminated alcohol are below 30%. The cleavage can be performed attemperatures, which are relatively low for cleavage reactions, of 150 to450° C., preferably at temperatures of 230 to 350° C. The cleavagetemperature includes all values and subvalues therebetween, especiallyincluding 200, 250, 300, 350 and 400° C. The conversions can beperformed at pressures of greater than 0.3 MPa, so that condensation ofthe isoolefins formed against cooling water is possible. The catalystfeatures a long lifetime. The catalyst does not contain any heavymetals, so that ecologically harmful substances occur neither in thecourse of its preparation nor in the course of its disposal. Variationof the content of alkaline earth metal oxide allows the activity to beadjusted optimally for each reactant.

The process according to the present invention and the inventivecatalysts are described by way of example below without any intentionthat the present invention be restricted to these exemplary embodiments.When ranges, general formulae or compound classes are specified below,these are intended to encompass not only the corresponding ranges orgroups of compounds which are mentioned explicitly but also allsub-regions or sub-groups of compounds which can be obtained byexcluding individual values (ranges) or compounds.

The process according to the present invention for preparing a catalystwhich, in a formal sense, comprises 0.1 to 20% by mass of alkali metaland/or alkaline earth metal oxide, 0.1 to 99% by mass of aluminum oxideand 0.1 to 99% by mass of silicon dioxide, comprises the steps of

a) treating an aluminosilicate with an aqueous alkali metal and/oralkaline earth metal salt solution under acidic conditions and

b) calcining the aluminosilicate treated with aqueous alkali metaland/or alkaline earth metal salt solution.

The amount of alkali metal and/or alkaline earth metal oxide includesall values and subvalues therebetween, especially including 0.5, 1, 2,4, 6, 8, 10, 12, 14, 16, and 18% by mass. The amount of aluminum oxideincludes all values and subvalues therebetween, especially including0.5, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80,85, 90 and 95% by mass. The amount of silicon dioxide includes allvalues and subvalues therebetween, especially including 0.5, 1, 5, 10,15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90 and 95%by mass.

Preference is given to preparing a catalyst which, in a formal sense,has a content of magnesium oxide of 0.5 to 20% by mass, a content ofaluminum oxide of 4 to 30% by mass and a content of silicon dioxide of60 to 95% by mass.

In the context of the present invention, aluminosilicates shall beunderstood to mean compounds which are composed, in a formal sense,essentially of contents of aluminum oxide (Al₂O₃) and silicon dioxide(SiO₂). The aluminosilicates used in the process according to thepresent invention may have 1 to 99% by mass of silicon dioxide and 1 to99% by mass of aluminum oxide (Al₂O₃). The inventive aluminosilicatesmay also contain small contents of alkali metal or alkaline earth metaloxides. In the process according to the present invention, thealuminosilicates used may also be zeolites, for example zeolite A, X, Y,USY or ZSM-5, or amorphous zeolites (for example MCM 41 from Mobil Oil).The composition of the aluminosilicates used or of the resultingcatalysts can be determined, for example, by classical analysis, fusionwith borax and XFA (X-ray fluorescence analysis), energy-dispersiveX-ray analysis, flame spectroscopy (Al and Mg, not Si), wet digestionand subsequent ICP-OES (optical emission spectrometry with inductivelycoupled high-frequency plasma) or atomic absorption spectroscopy.Preference is given to determining the composition by fusion with boraxand subsequent XFA or by wet digestion and subsequent ICP-OES.

The aluminosilicates used in the process according to the presentinvention may be amorphous or crystalline. Suitable commercialaluminosilicates which can be used as starting materials in the processaccording to the present invention are, for example, aluminosilicateswhich have been prepared by precipitation, gelation or pyrolysis.

In the process according to the present invention, preference is givento using aluminosilicates which have 5 to 40% by mass, preferably 10 to35% by mass, of aluminum oxide, and 60 to 95% by mass, preferably 65 to90% by mass, of silicon dioxide (based on the dry mass; treatment:calcination at 850° C. for 1 h). A particularly preferredaluminosilicate which can be used in the process according to thepresent invention has a formal content of Al₂O₃ of 10 to 15% by mass anda content of silicon dioxide of 73 to 78% by mass. Such analuminosilicate is supplied by Grace Davison under the name Davicat O701.

The aluminosilicate can be used in the process according to the presentinvention in a wide variety of different forms. For instance, thealuminosilicate can be used in the form of shaped bodies, for exampletablets, pellets, granule, strands or extrudates. The aluminosilicatecan also be used in the form of aluminosilicate powder. The startingmaterial used may be powders with different mean particle size anddifferent particle size distribution. In the process according to thepresent invention, preference is given to using an aluminosilicatepowder in which 95% of the particles have a mean particle size of 5 to100 μm, preferably 10 to 30 μm and more preferably 20 to 30 μm. Theparticle size can be determined, for example, by laser diffraction witha particle analyser from Malvern, for example the Mastersizer 2000.

Process Step a)

In step a), an alkali metal salt solution and/or an alkaline earth metalsalt solution can be used. It is also possible in step a) to use a saltsolution which comprises one or more salts of alkali metals and ofalkaline earth metals. To prepare the aqueous salt solutions, alkalimetal and/or alkaline earth metal compounds which are water-soluble orare converted to water-soluble compounds by adding an acid may be used.The salts used are preferably the nitrates of the alkali metals oralkaline earth metals.

The acidic aqueous alkali metal and/or alkaline earth metal saltsolution used preferably has a pH of less than 6, preferably of lessthan 6 to 3 and more preferably of 5.5 to 3.5. The pH can be determined,for example, with the aid of a glass electrode or indicator paper. Whenthe salt solution has a pH which is greater than or equal to 6, the pHcan be adjusted by adding an acid, preferably the acid whose alkalimetal and/or alkaline earth metal salt is present in the solution. Whenthe alkali metal and/or alkaline earth metal salt solution comprises thenitrates as salts, preference is given to using nitric acid as the acid.

The alkali metal salt solution used in the process according to thepresent invention is preferably a magnesium or calcium salt solution.Preference is given to using magnesium salt solutions which, asmagnesium salts, comprise the salts of strong mineral acids, for examplemagnesium nitrate hexahydrate or magnesium sulphate heptahydrate. Thecalcium salt used may, for example, be calcium nitrate tetrahydrate. Inthe process according to the present invention, preference is given tousing a magnesium salt solution and particular preference to using amagnesium nitrate (hexahydrate) solution.

When the alkali metal and/or alkaline earth metal salt solution used instep a) comprises a magnesium salt, the magnesium content of thesolution is preferably 0.1 to 3 mol/l, preferably 0.5 to 2.5 mol/l. Themagnesium content of the solution includes all values and subvaluestherebetween, especially including 0.5, 1, 1.5, 2, 2.5 mol/l.

The treatment in step a) can be effected in various ways which aresuitable for contacting the aluminosilicate with the alkali metal and/oralkaline earth metal salt solution. Possible treatment methods are, forexample, impregnation, saturation, spraying or immersing thealuminosilicate with the alkali metal and/or alkaline earth metal saltsolution. It may be advantageous when the treatment of thealuminosilicate is effected in such a way that the alkali metal and/oralkaline earth metal salt solution can act on the aluminosilicate for atleast 0.1 to 5 h, preferably 0.5 to 2 h. Such an action time may beadvantageous especially when the treatment is effected by simplesaturation. The treatment time includes all values and subvaluestherebetween, especially including 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5h.

In a preferred embodiment of the inventive step a) of the processaccording to the present invention, the treatment of aluminosilicate,especially aluminosilicate shaped bodies, with the alkali metal and/oralkaline earth metal salt solution can be effected, for example, byvacuum impregnation in a vacuum impregnation unit suitable therefor. Inthis type of treatment, the aluminosilicate in the vacuum impregnationunit is first evacuated. Subsequently, the alkali metal and/or alkalineearth metal salt solution is sucked in up to above the upper edge of thesupport bed, so that the entire aluminosilicate is covered with thesolution. After an action time which is preferably 0.1 to 10 h,preferentially 0.5 to 2 h, the solution which has not been taken up bythe support is discharged. The action time includes all values andsubvalues therebetween, especially including 0.5, 1, 1.5, 2, 2.5, 3,3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9 and 9.5 h.

In a further preferred embodiment of the inventive step a) of theprocess according to the present invention, the treatment ofaluminosilicate, especially aluminosilicate shaped bodies, with thealkali metal and/or alkaline earth metal salt solution can be effected,for example, by spraying or immersing the aluminosilicate. The sprayingor immersion of the aluminosilicate with the alkali metal and/oralkaline earth metal salt solution is preferably effected by spraying orpouring the solution onto the aluminosilicate rotating in a drum. Thetreatment can be effected in one step, i.e. the entire amount of alkalimetal and/or alkaline earth metal salt solution is added at the start tothe aluminosilicate in one step. However, the salt solution can also bemetered in by spraying or immersion in small portions, the period ofaddition being preferably 0.1 to 10 h and preferentially 1 to 3 h. Theamount of salt solution is preferably such that the entire solution ofthe aluminosilicate is taken up. The period of addition includes allvalues and subvalues therebetween, especially including 0.5, 1, 1.5, 2,2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9 and 9.5 h.

Saturation in particular, but also spraying or immersion, can beperformed in customary industrial apparatus, for example conical mixersor intensive mixers, as supplied, for example, by Eirich.

The treatment of the aluminosilicate with the alkali metal and/oralkaline earth metal salt solution in step a) can be effected in onestep or in a plurality of partial steps. In particular, it is possibleto perform the treatment in two or more partial steps. In each of theindividual partial steps, the same alkali metal and/or alkaline earthmetal salt solution can be used in each case, or else an alkali metaland/or alkaline earth metal salt solution of different concentrationand/or composition can be used in each partial step. For example,initially only a portion of the alkali metal and/or alkaline earth metalsalt solution can be added to the aluminosilicate and, optionally afterintermediate drying, the remaining amount of the salt solution used canbe added at the same temperature or a different temperature. However, itis also possible to treat the aluminosilicate with different alkalimetal and/or alkaline earth metal salt solutions (differentconcentration and/or composition) at the same or a differenttemperature. It is not only possible that step a) is performed in two ormore substeps. It is likewise possible that the process has a pluralityof steps a). In this case too, identical or different alkali metaland/or alkaline earth metal salt solutions in relation to concentrationand/or composition can be used in the different steps a).

The treatment in step a) can be performed preferably at a temperature of10 to 120° C., preferentially of 10 to 90° C., more preferably of 15 to60° C. and most preferably at a temperature of 20 to 40° C. Thetemperature includes all values and subvalues therebetween, especiallyincluding 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85,90, 95, 100, 105, 110, 115° C.

It may be advantageous when one or more additives is/are added oradmixed to the aluminosilicate or to the alkali metal and/or alkalineearth metal salt solution in step a). Such additives may, for example,be binders, lubricants or shaping assistants. A suitable binder may, forexample, be boehmite or pesudoboehmite, as supplied, for example, underthe name Disperal (boehmite having a formal Al₂O₃ content of approx. 77%by mass) by Sasol Deutschland GmbH. When boehmite, especially Disperal,is added as a binder, it is preferably added as a gel which can beobtained, for example, by stirring 197 parts by mass of Disperal into803 parts by mass of 1.28% by mass aqueous nitric acid, stirringthoroughly at 60° C. for 3 h, cooling to room temperature and replacingany evaporated water. The shaping assistants used may, for example, besilicas, especially pyrogenic silicas, as sold, for example, by DegussaAG under the name Aerosil, bentonites, clays, kaolin, kaolinite, ballclay and other substances familiar for this purpose to those skilled inthe art. The lubricants added, whose use may be advantageous forimproved tabletting, may, for example, be graphite.

One or more of the abovementioned additives may be added in step a) invarious ways. In particular, the addition can be effected during thetreatment of the aluminosilicate with the alkali metal and/or alkalineearth metal salt solution. For example, aluminosilicate, additive andalkali metal and/or alkaline earth metal salt solution can be chargedinto an industrial apparatus and then mixed intimately. Anotherpossibility is to first mix the aluminosilicate with the additive andthen to add the alkali metal and/or alkaline earth metal salt solution.In a further variant, additive and alkali metal and/or alkaline earthmetal salt solution can be metered simultaneously to thealuminosilicate. The addition can be effected in each case in one batch,in portions or by spraying. The addition time is preferably less than 5h, preferentially less than 3 h. It may be advantageous to continue tomix the mixture for 0.1 to 10 h, preferably for 0.5 to 3 h. The mixingtime includes all values and subvalues therebetween, especiallyincluding 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5,8, 8.5, 9 and 9.5 h.

Process Step b):

The process according to the present invention has at least one processstep b) in which the aluminosilicate treated with alkali metal and/oralkaline earth metal salt solution is calcined. The calcination iseffected preferably in a gas stream, for example in a gas stream whichcomprises, for example, air, nitrogen, carbon dioxide and/or one or morenoble gases, or consists of one or more of these components. Preferenceis given to effecting the calcining using air as the gas stream.

The calcination in the inventive process step b) is performed preferablyat a temperature of 200 to 1000° C., preferably of 300 to 800° C. Thecalcination is effected preferably for a time of 0.1 to 10 hours,preferably 1 to 5 hours. Particular preference is given to performingthe calcination at a temperature of 200 to 1000° C., preferably 300 to800° C., for 0.1 to 10 hours, preferably 1 to 5 hours. The temperatureincludes all values and subvalues therebetween, especially including300, 400, 500, 600, 700, 800 and 900° C. The calcinations time includesall values and subvalues therebetween, especially including 0.5, 1, 1.5,2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9 and 9.5 h.

The industrial calcination can preferably be performed in a shaft oven.However, the calcination can also be performed in other known industrialapparatus, for example fluidized bed calciners, rotary tube ovens ortray ovens.

Process Step c):

It may be advantageous when a step c), in which the aluminosilicatetreated with alkali metal and/or alkaline earth metal salt solution isdried, is performed between steps a) and b). The drying in step c) canbe effected at a temperature of 100 to 140° C. The temperature includesall values and subvalues therebetween, especially including 110, 120,and 130° C. The drying is preferably effected in a gas stream. Thedrying can be performed, for example, in a gas stream which comprises,for example, air, nitrogen, carbon dioxide and/or one or more noblegases, or consists of one or more of these components. The intermediatedrying step after the treatment with alkali metal and/or alkaline earthmetal salt solution and before the calcining can achieve the effect thatno large amounts of steam are released in the course of calcination. Inaddition, the drying can prevent water which evaporates spontaneously inthe course of calcining from destroying the shape of the catalyst.

Depending on the desired shape in which the catalyst is to be present,it may be advantageous to adjust the preparation process appropriatelyby additional process steps.

When, for example, pulverulent catalyst is to be prepared by the processaccording to the present invention, the aluminosilicate can be used inthe form of aluminosilicate powder and, for example, treated with thealkali metal and/or alkaline earth metal salt solution (for example byimpregnation), for example in a conical mixer, optionally dried and thencalcined. However, a pulverulent catalyst can also be prepared byprocessing a shaped catalyst body to give a pulverulent catalyst bygrinding and screening.

The shaped catalyst bodies may be present, for example, in the form ofextrudates, spheres, pellets or tablets. In order to arrive at theshaped catalyst (shaped catalyst bodies), depending on the particularshaping variant, it is possible to perform further process steps, forexample shaping, grinding or screening, in addition to the process stepsof treatment, drying, calcination. Shaping assistants can be introducedat various points in the process. The shaped catalyst bodies can beprepared in various ways:

In a first variant, shaped catalyst bodies, especially inventive shapedcatalyst bodies, can be obtained by treating shaped aluminosilicatebodies with an aqueous alkali metal and/or alkaline earth metal saltsolution, optionally drying and then calcining.

In a second embodiment, a shaped catalyst body, especially an inventiveshaped catalyst body, can be obtained by first treating analuminosilicate powder with an acidic aqueous alkali metal and/oralkaline earth metal salt solution, then optionally drying andsubsequently calcining it, and subsequently processing the resultingcatalyst powder by processes customary in industry, for examplecompaction, extrusion, pelletization, tabletting, granulation or coatingto give shaped catalyst bodies. Additives required for the shaping, forexample binders or further assistants, can be added at various points inthe preparation process, for example in process step a). When a shapedbody is prepared from an aluminosilicate powder as a starting material,it is possible to start from powders with different mean particle sizeand different particle size distribution. For the preparation of shapedbodies, preference is given to using an aluminosilicate powder in which95% of the particles have a particle size of 5 to 100 μm, preferably 10to 30 μm and more preferably 20 to 30 μm (determined by laserdiffraction; see above). The particle size includes all values andsubvalues therebetween, especially including 10, 15, 20, 25, 30, 35, 40,45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 μm.

In a third embodiment of the process according to the present invention,pellets of the catalyst, especially of the inventive catalyst, can beobtained by, in process step a), treating an aluminosilicate powder withan aqueous acidic alkali metal and/or alkaline earth metal saltsolution, optionally drying (process step c)) and then calcining inprocess b), and pelletizing the catalyst powder thus obtained withaddition of binder, for example in an Eirich mixer, and drying theresulting pellets in a further process step c) and then calcining themin a further process step b).

In a fourth embodiment of the process according to the presentinvention, pellets of the catalyst, especially of the inventivecatalyst, can be obtained by, in process step a), mixing analuminosilicate powder, binder and acidic aqueous alkali metal and/oralkaline earth metal salt solution, and pelletizing the aluminosilicatepowder thus treated, for example in an Eirich mixer, and drying theresulting moist pellets in process step c) and then calcining them in agas stream in process step b).

In a fifth embodiment of the process according to the present invention,tablets of the catalyst, especially of the inventive catalyst, can beobtained by, in process step a), mixing an aluminosilicate powder,binder, optionally lubricant and acidic aqueous alkali metal and/oralkaline earth metal salt solution, and pelletizing the aluminosilicatepowder thus treated, for example in an Eirich mixer, to givemicropellets, preferably having a mean diameter of 0.5 to 10 mm,preferably 1 to 5 mm and more preferably of 1 to 3 mm (the particle sizecan be determined, for example, by screen analysis), and drying theresulting moist pellets in process step c) and then optionally calciningthem in a gas stream in process step b). The resulting pellets may then,unless already done in process step a), be mixed with a lubricant, forexample graphite, and then tabletted on a commercial tabletting press,for example a rotary tabletting press. The tablets may then, if processstep b) is yet to be performed, be calcined in process step b), oroptionally post-calcined.

In a sixth embodiment of the process according to the present invention,tablets of the catalyst, especially of the inventive catalyst, can beobtained by grinding preshaped shaped catalyst bodies, as can beobtained, for example, as pellets in embodiment three or four, andscreening the granule/powder obtained, so as to obtain a tablettablegranule of catalyst, and adding lubricants to this granule. The granulethus prepared can then be tabletted. The tablets may then, if processstep b) is yet to be performed, be calcined in process step b). Theaddition of a lubricant can be dispensed with when a lubricant hasalready been added in the course of preparation of the pellets, forexample in process step a).

In a seventh embodiment of the process according to the presentinvention, materials/supports coated with the catalyst, especially withthe inventive catalyst, can be prepared. In this embodiment, a catalystpowder is first prepared by, in process a), treating an aluminosilicatepowder with an acidic aqueous alkali metal and/or alkaline earth metalsalt solution, optionally drying (process step c)) and optionallycalcining (process step b)). The catalyst powder thus obtained is thensuspended in a suspension medium, for example water or alcohol, forwhich a binder can optionally be added to the suspension. The suspensionthus prepared can then be applied to any material. The application isfollowed by optional drying (process step c)) and then calcining(process step b)). In this way, materials/supports coated with thecatalyst, especially with the inventive catalyst, can be provided. Suchmaterials/supports may, for example, be metal plates or fabric, as canbe used as internals in reactors or columns, especially reactivedistillation columns, or else metal, glass or ceramic spheres, orspheres of inorganic oxides.

In an eighth embodiment of the process according to the presentinvention, extrudates of the catalyst, especially of the inventivecatalyst, can be obtained by, in process step a), mixing analuminosilicate powder, acidic aqueous alkali metal and/or alkali metalsalt solution, binder, for example Disperal, and further shapingassistants customary for extrusion, for example clays such as bentoniteor attapulgite, in a kneader or Eirich mixer, and extruding them in anextruder to give extrudates, preferably having a mean diameter of 0.5 to10 mm, preferentially of 1 to 5 mm and more preferably of 1 to 3 mm, anddrying the resulting moist extrudates optionally in process step c) andthen calcining them in a gas stream in process step b). The meandiameter includes all values and subvalues therebetween, especiallyincluding 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8,8.5, 9 and 9.5 mm.

The catalysts obtainable by the process according to the presentinvention are especially those which consist, in a formal sense, ofaluminum oxide (Al₂O₃) and silicon dioxide (SiO₂) and at least one oxidefrom the group of alkali metal and alkaline earth metal oxides. Theaverage mass fraction of aluminum oxide in these catalysts is preferably1 to 99% by weight, that of silicon dioxide preferably 1 to 99% byweight and the mass fraction of the alkali metal and alkaline earthmetal oxides in total preferably 0.1 to 30% by weight. Such catalystspreferably have magnesium as the alkaline earth metal, in particular asthe sole alkaline earth metal. The average mass fraction of aluminumoxide includes all values and subvalues therebetween, especiallyincluding 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80,85, 90, 95% by weight. The mass fraction of silicon dioxide includes allvalues and subvalues therebetween, especially including 5, 10, 15, 20,25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95% by weight.The mass fraction of the alkali metal and alkaline earth metal oxidesincludes all values and subvalues therebetween, especially including0.5, 1, 5, 10, 15, 20, 25% by weight.

The inventive catalyst which, in a formal sense, comprises alkali metaland/or alkaline earth metal oxide, aluminum oxide and silicon dioxidefeatures, in a formal sense, a content of alkali metal and/or alkalineearth metal oxides (total of the alkali metal and/or alkaline earthmetal oxides present in the catalyst) of 0.5 to 20% by mass, a contentof aluminum oxide (Al₂O₃) of 4 to 30% by mass and a content of silicondioxide of 60 to 95% by mass. The inventive catalyst may, for example,be obtained by the above-described process according to the presentinvention. The content of alkali metal and/or alkaline earth metaloxides includes all values and subvalues therebetween, especiallyincluding 1, 2, 4, 6, 8, 10, 12, 14, 16, 18% by mass. The content ofaluminum oxide includes all values and subvalues therebetween,especially including 5, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and28% by mass. The content of silicon dioxide includes all values andsubvalues therebetween, especially including 65, 70, 75, 80, 85 and 90%by mass.

The inventive catalyst preferably contains an alkaline earth metaloxide, preferably magnesium oxide. The inventive catalyst morepreferably contains magnesium oxide as the sole alkaline earth metaloxide. It may be advantageous when the catalyst comprises an alkalimetal oxide in addition to the alkaline earth metal oxide. This may, forexample, be selected from Na₂O or K₂O. The inventive catalyst preferablycomprises Na₂O as the alkali metal oxide.

When the inventive catalyst comprises magnesium oxide, it preferably hasa content of magnesium oxide of 0.5 to 20% by mass, preferably of 5 to15% by mass and more preferably of 10 to 15% by mass, and a content ofaluminum oxide of 4 to 30% by mass, preferably of 10 to 20% by mass, anda content of silicon dioxide of 60 to 95% by mass, preferably of 70 to90% by mass. The content of magnesium oxide includes all values andsubvalues therebetween, especially including 1, 2, 4, 6, 8, 10, 12, 14,16, 18% by mass. The content of aluminum oxide includes all values andsubvalues therebetween, especially including 5, 6, 8, 10, 12, 14, 16,18, 20, 22, 24, 26, and 28% by mass. The content of silicon dioxideincludes all values and subvalues therebetween, especially including 65,70, 75, 80, 85 and 90% by mass.

The inventive catalyst preferably has a BET surface area (determined byvolumetric means with nitrogen to DIN ISO 9277) of 200 to 450 m²/g,preferably of 200 to 350 m²/g. The BET surface area includes all valuesand subvalues therebetween, especially including 300, 350 and 400 m²/g.When the inventive catalyst is applied as an active composition on asupport, only the active composition has a BET surface area in the rangespecified. The material composed of catalyst and support may, incontrast, depending on the properties of the support, have asignificantly different BET surface area, especially a smaller BETsurface area. The pore volume of the inventive catalyst is preferably0.5 to 1.3 ml/g, preferentially 0.65 to 1.1 ml/g. The pore volume ispreferably determined by the cyclohexane method. The pore volumeincludes all values and subvalues therebetween, especially including0.6, 0.8, 1.0 and 1.2 ml/g.

In this method, the sample to be tested is first dried at 110° C. toconstant weight. Subsequently, approx. 50 ml of the sample weighedaccurately to 0.01 g are introduced into a cleaned impregnation tubedried to constant weight, which has an outlet orifice with aground-glass tap at the lower end. The outlet orifice is covered with asmall piece of polyethylene, which prevents blockage of the outletorifice by the sample. After the impregnation tube has been filled withthe sample, the tube is carefully sealed air-tight. Subsequently, theimpregnation tube is connected to a water jet pump, the ground-glass tapis opened and the water jet is used to establish a vacuum in theimpregnation tube of 20 mbar. The vacuum can be checked on a parallelvacuum meter. After 20 min, the ground-glass tap is opened and theevacuated impregnation tube is subsequently connected to a cyclohexanereceiver in which an accurately measured volume of cyclohexane isinitially charged, such that opening of the ground-glass tap results insuction of cyclohexane from the receiver into the impregnation tube. Theground-glass tap remains open until the entire sample has been floodedwith cyclohexane. Subsequently, the ground-glass tap is closed again.After 15 min, the impregnation tube is aerated cautiously and theunabsorbed cyclohexane is discharged into the receiver. Cyclohexaneadhering in the impregnation tube or in the outlet orifice or theconnection to the cyclohexane receiver can be conveyed via the aerationline into the receiver by a single cautious pressure impulse from asuction ball. The volume of the cyclohexane present in the receiver isnoted. The pore volume is determined from the absorbed volume ofcyclohexane, which is determined from the cyclohexane volume in thereceiver before the measurement minus the cyclohexane volume in thereceiver after the measurement, divided by the mass of the sampleanalysed.

The mean pore diameter (preferably determined on the basis of DIN 66133)of the inventive catalyst is preferably 5 to 20 nm, preferably 8 to 15nm. The mean pore diameter includes all values and subvaluestherebetween, especially including 6, 8, 10, 12, 14, 15, 16, 18 nm. Morepreferably, at least 50%, preferably over 70%, of the total pore volume(sum of the pore volume of the pores having a pore diameter of greaterthan or equal to 3.5 nm determined by mercury porosimetry to DIN 66133)of the catalyst is accounted for by pores having a diameter of 3.5 to 50nm (mesopores).

The inventive catalysts may have different dimensions, especially adimension of 10 μm to 10 mm. The dimension includes all values andsubvalues therebetween, especially including 100, 500, 1000, 1500, 2000,2500, 3000, 3500, 4000, 4500, 5000, 5500, 6000, 6500, 7000, 7500, 8000,8500, 9000 and 9500 μm. For use of the catalysts in a fluidized bedreactor, the mean particle size (determined by means of laserdiffraction on a Mastersizer 2000 from Malvern) of the catalyst ispreferably 10 to 200 μm, preferentially 50 to 150 μm. The mean particlesize includes all values and subvalues therebetween, especiallyincluding 20, 40, 60, 80, 100, 120, 140, 160, 180 μm. For the use of thecatalyst in fixed bed reactors, it is preferably present in the form ofa shaped body, for example as a strand, pellet, tablet, sphere orgranule, and preferably has dimensions (longest dimension or diameter)of 0.5 to 10 mm, preferably of 1 to 5 mm.

The inventive catalyst may also be applied on a support, for example ametal, plastic or ceramic support, preferably on a support which isinert in relation to the reaction in which the catalyst is to be used.In particular, the inventive catalyst may also be applied to a metalsupport, for example a metal plate or a metal fabric. Such supportsprovided with the inventive catalyst may be used, for example, asinternals in reactors or reactive distillation columns. The supports mayalso be metal, glass or cereamic spheres or spheres of inorganic oxides.When the inventive catalyst is applied on an inert support, the mass andcomposition of the inert support is not taken into account in thedetermination of the composition of the catalyst.

The inventive catalyst or a catalyst prepared by the process accordingto the present invention can be used as a catalyst for numerousreactions. In particular, the inventive catalyst or a catalyst preparedby the process according to the present invention can be used in aprocess for preparing isoolefins having 4 to 6 carbon atoms of theformula I

by catalytic gas phase cleavage of a starting compound of the formula II

to give a compound of the formula I and a compound of the formula III

R—OH  III

where, in the formulae Ito III, the R radical is H or an alkyl radicalhaving 1 or 2 carbon atom(s), the R¹ radical is H or a methyl or ethylradical, and the R² and R³ radicals are each methyl or ethyl radicals,where the R² and R³ radicals may be the same or different, at atemperature of 110 to 450° C. and a pressure of 0.1 to 2 MPa. Thetemperature includes all values and subvalues therebetween, especiallyincluding 150, 200, 250, 300, 350 and 400° C. The pressure includes allvalues and subvalues therebetween, especially including 0.5, 1 and 1.5MPa.

The compounds of the formula II used may, for example, be tertiaryalcohols having from 4 to 6 carbon atoms. In such a process according tothe present invention, the compound II cleaved is preferablytert-butanol (TBA) to isobutene as the compound of the formula I andwater as the compound III.

The TBA which can be used in the cleavage process can stem from varioussindustrial processes. One of the most important is the reaction ofisobutenic C₄ hydrocarbon mixtures with water. Processes for preparingTBA are published, for example, in the patents DE 103 30 710 and U.S.Pat. No. 7,002,050. TBA can be used in pure form, as a TBA/waterazeotrope or as another TBA-water mixture.

In a cleavage process according to the present invention, preference isgiven to cleaving a compound of the formula II in which R is a methyl,ethyl or propyl radical. Alkyl tert-alkyl ethers which can be used inthe cleavage process according to the present invention are, forexample, methyl tert-butyl ether, ethyl tert-butyl ether or tert-amylmethyl ether (TAME). In the case of the inventive use of the inventivecatalyst or of the catalyst prepared in accordance with the presentinvention, particular preference is given to cleaving methyl tert-butylether to isobutene and methanol or ethyl tert-butyl ether to isobuteneand ethanol.

In the cleavage process according to the present invention, it ispossible to use ATAEs which may stem from a wide variety of differentprocesses. One process for preparing MTBE is described, for example, inDE 101 02 062. Processes for preparing ETBE are published, for example,in DE 10 2005 062700, DE 10 2005 062722, DE 10 2005 062699 or DE 10 2006003492.

The inventive cleavage in the gas phase over the inventive catalyst isperformed preferably at a temperature of 150 to 400° C. When thestarting material used is MTBE, the cleavage of MTBE to isobutene andmethanol is preferably performed at a temperature of 180 to 400° C.,more preferably of 230 to 350° C.

The cleavage process according to the present invention is preferablyperformed at a reaction pressure of 0.1 to 1 MPa. The reaction pressureincludes all values and subvalues therebetween, especially including0.2, 0.4, 0.5, 0.6, 0.8 MPa. When isobutene is a product, it may beadvantageous to perform the cleavage process according to the presentinvention at a pressure of 0.2 to 1 MPa, preferably of 0.5 to 0.8 MPa.This is advantageous especially because isobutene can be condensedagainst cooling water at these pressures.

The specific catalyst hourly space velocity (WHSV; grams of reactant atroom temperature per gram of catalyst per hour) in the cleavage processaccording to the present invention is preferably 0.1 to 100 h⁻¹,preferably 0.5 to 30 h⁻¹. The specific catalyst hourly space velocityincludes all values and subvalues therebetween, especially including0.5, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80,85, 90, 95 h⁻¹. When the starting material used is MTBE, the cleavage ofMTBE to isobutene and methanol is preferably performed at a WHSV of 0.1to 100 h⁻¹, more preferably of 0.25 to 25 h⁻¹.

In order to minimize the workup complexity of the cleavage productmixture, high conversions for straight pass are preferably pursued. Theprocess according to the present invention is preferably performed insuch a way that the conversions of compounds to be cleaved are over 70%,preferably over 80% and more preferably over 90% to 100%. When thereactants contain troublesome secondary components, it may beappropriate to restrict the conversion. When, for example, the feedstockmixture also contains 2-methoxybutane in addition to the MTBE to becleaved, it may be necessary to reduce the conversion in straight passin order not to exceed a defined ratio of linear butenes to isobutene inthe reaction mixture. It may thus be advantageous to restrict thepermissible conversion of MTBE the higher the content of 2-methoxybutaneis in the feedstock mixture comprising MTBE. The restriction of theconversion can be achieved, for example, by increasing the WHSV and/orlowering the reaction temperature.

The selectivity of isoolefin formation in the process according to thepresent invention is preferably over 98%, preferentially over 99%. Theselectivity for the formation of alcohols in the process according tothe present invention in the case of ATAE cleavage, especially in thecase of ATBE cleavage, is over 95%, especially over 99%.

The cleavage product mixture can be worked up by known industrialprocesses. Unconverted reactant can be recycled into the cleavage,optionally after partial discharge or purification.

The isoolefins obtained may be utilized as described in theintroduction. Isobutene prepared by the cleavage process according tothe present invention can be used in particular for the preparation ofbutyl rubber, polyisobutylene, isobutene oligomers, branched C₅aldehydes, C₅ carboxylic acids, C₅ alcohols, C₅ olefins,tert-butylaromatics and methacrylic acid and esters thereof.

The alcohols obtained in the cleavage of ATAE can be used again afterprocessing, for example for the synthesis of ATAE.

Having generally described this present invention, a furtherunderstanding can be obtained by reference to certain specific exampleswhich are provided herein for purposes of illustration only, and are notintended to be limiting unless otherwise specified.

EXAMPLES Example 1 Preparation of a Shaped Aluminosilicate Body

500 g of aluminosilicate powder (manufacturer: Grace Davison, Type:Davicat O 701, formal Al₂O₃ content: 13% by mass, formal SiO₂ content:76% by mass, formal Na₂O content: 0.1% by mass, ignition loss at 850°C.: approx. 11%), 363 g of Disperal gel (formal Al₂O₃ content: 15.6%),(which was obtained by stirring 197 g of Disperal, a boehmite having aformal Al₂O₃ content of 77% by mass from Sasol Deutschland GmbH, into803 g of 1.28% by mass aqueous nitric acid, subsequently stirringthoroughly, in the course of which the gel which formed was shearedconstantly and thus kept in a free-flowing state, in a covered vessel at60° C. for 3 h, cooling the gel to room temperature and replacement ofany evaporated water) and 370 g of demineralized water (DM water) wereinitially mixed thoroughly with one another in an intensive mixer fromEirich. Subsequently, the mixture was pelletized in the intensive mixerfrom Eirich, to obtain uniformly round pellets with a diameter ofapprox. 1 to 3 mm within 30-40 minutes. The moist pellets were firstdried in an air stream at 120° C. and then heated at 2 K/min to 550° C.and calcined in an air stream at this temperature for 10 h. Thealuminosilicate pellets thus prepared contained, in a formal sense, 76%by mass of Al₂O₃ and 24% by mass of SiO₂. In addition, the catalystprepared contained 0.12% by mass of sodium compounds (calculated assodium oxide). The composition of the aluminosilicate pellets wascalculated from the amount and the composition of the startingsubstances. The aluminosilicate pellets had a pore volume, determined bythe above-described cyclohexane method, of 1.15 ml/g.

Example 2 Preparation of a Shaped Catalyst (According to the PresentInvention)

An impregnation solution having a magnesium content of 4.7% by mass wasprepared from DM water and magnesium nitrate hexahydrate. The pH of thissolution was 5.1. By means of vacuum impregnation, a screened-outfraction of the aluminosilicate support prepared in Example 1 (diameter:1.0 mm-2.8 mm) was impregnated with the acidic magnesium nitratesolution. To this end, the pellets were introduced into a glass tubewhich was evacuated for about 30 min (water-jet pump vacuum of approx.25 hPa). Subsequently, the impregnation solution was sucked in from thebottom up to above the upper edge of the solid-state bed. After anaction time of about 15 minutes, the solution which had not been takenup by the support was discharged. The moist pellets were first dried toconstant weight in an airstream at 140° C. and then heated to 450° C. at3 K/min and calcined at this temperature for 12 h. The catalyst preparedconsisted, in a formal sense, of 68% by mass of silicon dioxide, of 21%by mass of aluminum oxide and of 11% by mass of magnesium oxide. Inaddition, the catalyst prepared contained 0.11% by mass of sodiumcompounds (calculated as sodium oxide). The composition of the catalystwas calculated from the amount and the composition of the startingsubstances, and the impregnation solution which had run off. The amountsof sodium were part of the aluminosilicate used in Example 1. The porevolume, determined by the above-described cyclohexane method, was 1.1ml/g.

Example 3 Preparation of a Pulverulent MgO/Al₂O₃/SiO₂ Catalyst (byImpregnation of an Aluminosilicate with a Magnesium Salt Solution)

500 g of an aluminosilicate powder (manufacturer: Grace Davison, type:Davicat O 701, Al₂O₃ content: 13% by weight, SiO₂ content: 76% byweight, Na₂O content: 0.1% by weight, ignition loss at 850° C.: 11%,particle size d₅₀=20 μm, determined by means of laser diffraction on aMastersizer 2000 from Malvern) with a pore volume of 1.1 ml/g were mixedin a conical mixer with 550 ml of an acidic magnesium nitrate solution(Mg content: 4.7% by mass, pH=5.1) and mixed for 20 minutes. The moistpowder was then first dried to constant weight at 140° C. in anairstream in a fluidized bed oven and then calcined at 450° C. in amuffle furnace for 10 h. The content of the main components calculatedfrom the starting compounds (based on these main components) in thecatalyst calcined at 850° C. was 12% by mass of Al₂O₃, 77% by mass ofSiO₂, 11% by mass of MgO.

Example 4 Preparation of Catalyst Tablets

645 g of magnesium nitrate hexahydrate were dissolved in 355 g of DMwater. The pH of this solution was 3.5. 500 g of Disperal gel (formalAl₂O₃ content: 15.6% by weight) were stirred into this solution. Theresulting viscous, but still readily stirrable and pumpable, suspensionwas then premixed directly with 917 g of aluminosilicate powder(manufacturer: Grace Davison, type: Davicat O 701, Al₂O₃ content: 13% byweight, SiO₂ content: 76% by weight, Na₂O content: 0.1% by weight,ignition loss at 850° C.: 11%, particle size d₅₀=20 μm, determined bymeans of laser diffraction on a Mastersizer 2000 from Malvern) and 42 gof graphite (manufacturer: Edelgraphitgesellschaft Bonn, type: K 16) astabletting assistants. This afforded a mass which consists of loose,slightly moist agglomerates. This mass was then pelletized in anintensive mixer from Eirich. In order to obtain tablettable granule, thepelletization was controlled in such a way that predominantly roundedpellets having a diameter of less than 1 mm were formed. The moistpellets were then initially dried at 140° C. for 2 h and then calcinedat 450° C. in a rotary tube flowed through by air for 1 h.

The calcined pellets were then compressed with a tabletting press (fromRonchi) to give tablets having a diameter of 3 mm and a height of 4 mm.Subsequently, the tablets were post-calcined at 560° C. for 2 hours. Thenominal content in the tablet of Al₂O₃ was 19.7 parts by mass, that ofSiO₂ 70.8 parts by mass, that of MgO 10.0 parts by mass, that of Na₂O0.1 part by mass. The tablets had a BET surface area (to DIN ISO 9277)of 253 m²/g. The total pore volume determined by means of mercuryporosimetry on the basis of DIN 66133 (maximum measurement pressure: 400MPa) for pores having a diameter of 3.5 to 10 000 nm was 0.68 ml/g. Themacropore volume (pores having a diameter of 50-10 000 nm) was 0.1 ml/g(macropore content 15%) and the mesopore volume (pores with diameter of3.5 to 50 nm) was 0.57 ml/g (mesopore content 85%). The mean porediameter was 9.6 nm.

Example 5 Gas Phase Cleavage of MTBE to Isobutene and Methanol (inAccordance with the Present Invention)

The cleavage was performed in a tubular reactor with a heating jacketthrough which a heat carrier oil (Marlotherm SH from Sasol Olefins &Surfactants GmbH) flowed. The catalyst used was the catalyst prepared inExample 2. The reactant used was industrial MTBE (Driveron from OxenoOlefinchemie GmbH) having a purity of 99.7% by mass.

Before entry into the reactor, the MTBE was evaporated fully in anevaporator at 250° C. At a temperature of 250° C. (temperature of theMarlotherm in the feed of the reactor jacket) and a pressure of 7bar_(absolute) (bara), 500 g per hour of MTBE were passed through 282 gof catalyst, corresponding to a WHSV of 1.77 h⁻¹. The gaseous cleavageproduct mixture was partly condensed in a condenser. The resultingliquid phase was weighed and the volume of the gas phase was measured.The two phases were analysed by gas chromatography.

The cleavage was performed under the above conditions over a period ofmore than 2500 hours. The isobutene conversions, the selectivities ofisobutene formation (number of moles of isobutene formed relative tonumber of moles of MTBE converted) and the selectivities of methanolformation (number of moles of methanol formed relative to number ofmoles of isobutene converted) were calculated at various reaction timesfrom the composition of the cleavage product mixture, the reactant massand the mass of the cleavage product mixture. The resulting values werecompiled in Table 1 below.

TABLE 1 Conversion and selectivities of the cleavage of MTBE in Example5 Experimental duration (h) 51 99 495 1000 1511 1992 2497 MTBEconversion (%) 85.4 84.2 81.1 77.3 76.2 73.6 72.5 Isobutene 99.98 99.9899.97 99.97 99.97 99.97 99.97 selectivity (%) Methanol 97.1 97.4 97.897.8 97.7 97.8 97.7 selectivity (%)

The experiment shows that the inventive catalyst is very suitable forthe cleavage of MTBE to isobutene and methanol. The selectivities bothof isobutene formation and of methanol formation were excellent andremained virtually constant over the entire experimental duration. Theactivity of the catalyst decreased with increasing experimentalduration. However, after approx. 2500 hours, it was still 85% of theactivity of the catalyst after 50 h of experimental duration, so that itwas possible to assume a lifetime of the catalyst of well over one yearin an industrial production plant.

Example 6 Gas Phase Cleavage of MTBE to Isobutene and Methanol (inAccordance with the Present Invention)

In the same apparatus as in Example 5, MTBE was cleaved at 250° C. Thecatalyst used was 10 g of the tablets prepared in Example 4. Thethroughput was 100 g of MTBE per hour. This corresponded to a WHSV of 10h⁻¹. The experiment ran over 480 hours. The experimental results werecompiled in Table 2.

TABLE 2 Conversions and selectivities of the cleavage of MTBE accordingto Example 6 Experimental duration (h) 48 144 480 MTBE conversion (%)86.9 80.2 67.4 Isobutene selectivity (%) 99.7 99.8 99.9 Methanolselectivity (%) 99.27 99.29 99.12

The experiment shows that the tabletted catalyst from Example 4 has ahigher activity than the catalyst from Example 2. The selectivity ofmethanol formation is better and the isobutene formation is somewhatlower than in the case of the catalyst from Example 2, but stilloutstanding. This catalyst too is suitable for the industrialpreparation of isobutene by cleaving MTBE. Owing to the high activity ofthe catalyst, only a relatively small catalyst volume is required.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, thepresent invention may be practiced otherwise than as specificallydescribed herein.

German patent application 102006040432.7 filed Aug. 29, 2007, isincorporated herein by reference.

Numerous modifications and variations on the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the presentinvention may be practiced otherwise than as specifically describedherein.

1-17. (canceled)
 18. A process for preparing isoolefins having 4 to 6carbon atoms of the formula I

by catalytic gas phase cleavage of a starting compound of the formula II

to give a compound of the formula I and a compound of the formula IIIR—OH  III wherein, in the formulae I to III, the R radical is H or analkyl radical having 1 or 2 carbon atom(s), the R¹ radical is H or amethyl or ethyl radical, and the R² and R³ radicals are each methyl orethyl radicals, where the R² and R³ radicals may be the same ordifferent, at a temperature of 110 to 450° C. and a pressure of 0.1 to 2MPa, wherein the catalyst used in the gas phase cleavage comprises: (i)0.5 to 20% by mass of a member selected from the group consisting ofMgO, or a mixture of an alkaline earth metal oxide and MgO; (ii) 4 to30% by mass of aluminum oxide; and (iii) 60 to 95% by mass of silicondioxide; wherein a mean pore diameter of the catalyst is from 5 to 20nm.
 19. The process according to claim 18, wherein the compound IIcleaved is tert-butanol to isobutene as the compound of the formula Iand water as the compound III.
 20. The process according to claim 18,wherein a compound of the formula II in which R is a methyl, ethyl orpropyl radical is cleaved.
 21. The process according to claim 20,wherein methyl tert-butyl ether is cleaved to isobutene and methanol, orethyl tert-butyl ether to isobutene and ethanol.
 22. The processaccording to claim 18, wherein the catalyst comprises 5 to 15% by massof magnesium oxide, 10 to 20% by mass of aluminum oxide, and 70 to 90%by mass of silicon dioxide.
 23. The process according to claim 18,wherein the catalyst has a BET surface area of 200 to 450 m²/g.
 24. Theprocess according to claim 18, wherein the catalyst has a pore volume of0.5 to 1.3 ml/g.
 25. The process according to claim 18, wherein thecatalyst the mean pore diameter of the catalyst is 8 to 15 nm.
 26. Theprocess according to claim 18, wherein the catalyst has a longestdimension of from 10 μm to 10 mm.